ScrollView在Screenmanager类中不起作用

Question

boxlayout中的滚动视图不起作用，当我运行/打开时，屏幕变黑，可能我知道代码中有什么问题。

main.py

class TransLogs(Screen):
    layout = GridLayout(cols=3, spacing=10, size_hint_y=None)
    layout.add_widget(Label(text='Date',size_hint_y=None,height=40))

   layout.add_widget(Label(text='Operation',size_hint_y=None,height=40))
   layout.add_widget(Label(text='Amount',size_hint_y=None,height=40))
   cur.execute("select * from log")
   f=cur.fetchall()
   for i in f:
       for j in i:

   layout.add_widget(Label(text=str(j),size_hint_y=None,height=40))
   root = ScrollView(size_hint=(1, None), size=(Window.width, 
   Window.height))
   root.add_widget(layout)
pass